- Compared to the significance of said event, the act of other people "observing" that Kurisu is in fact, dead, is a very minor detail carrying a negligible world line divergence %

- Therefore, the possibility of other people having "observed" Kurisu's death can be ignored, as Kurisu alive or dead, won't be causing a time paradox.

.

Sorry I actually edited my post, as I remembered some details while writing it... >_< But the gist of it should be the same...

Spoiler for game:

..except for that Okarin, while being the deciding trigger, is not the only factor which decides the worldlines
The possibility that someone else observed Kurisu's death (fake or true) is indeed negligible in the world line divergence % as at that point, no one besides Okarin can cause a shift in the divergence % through their actions.
I think it is safe to say that the divergence shift in this case is solely dependent on

-Did Okarin send the d-mail?

-Is Kurisu dead or alive?

Read your edit. I think what I wrote in this post should serve as answer?
I won't edit this post, as the edits are getting confusing enough as it is. XD